Local networks often make use of a communication line, such as a communication bus, over which a set of nodes communicates. A driver module in a master node applies power to the line, the driver module being switched to produce step changes in the power in the line to transmit signals to receivers in remote slave nodes over the line. The switched power signal activates the multiplexed remote nodes connected to the line and the line also selectively transmits signals from the remote nodes back to a central processing unit.
Such a bus is used in automotive vehicles, for example, the bus comprising either a single line or a twisted pair of conductors in which the current flows, the close coupling between the pair of conductors reducing their sensitivity to electromagnetic interference (‘EMI’), that is to say reception of noise induced in the wires of the bus, and improving their electromagnetic compatibility (‘EMC’), that is to say the radiation of parasitic fields by the currents flowing in the wires of the bus; both are critical parameters, especially in automotive applications.
Historically, in automotive applications, functions such as door locks, seat positions, electric mirrors, and window operations have been controlled directly by electrical direct current delivered by wires and switches. Such functions may today be controlled by ECUs (Electronic Control Units) together with sensors and actuators in a multiplexed Controller Area Network (CAN). The Controller Area Network (CAN) standard (ISO 11898) allows data to be transmitted by switching a voltage, at a frequency of 250 kbauds to 1 Mbaud for example, to the multiplexed receiver modules over the twisted pair cable. The receiver modules may be actuators that perform a function, for example by generating mechanical power required, or sensors that respond to activation by making measurements and transmitting the results back to the ECU over the bus.
The CAN bus was designed to be used as a vehicle serial data bus, and satisfies the demands of real-time processing, reliable operation in a vehicle's EMI environment, is cost-effective, and provides a reasonable data bandwidth. However, connecting with the main body network directly via a CAN bus system can be expensive because of increased costs per node and because high overall network traffic can make management extremely difficult. To help reduce costs, the logical extension is to structure the network hierarchically.
A variant on the CAN standard is the LIN (Local Interconnect Network) sub-bus standard (see ISO 7498), which is an extension to the CAN bus, at lower speed and on a single wire bus, to provide connection to local network clusters. A LIN sub-bus system uses a single-wire implementation (enhanced ISO9141), which can significantly reduce manufacturing and component costs. Component costs are further reduced by self-synchronization, without crystal or ceramics resonator, in the slave node. The system is based on common Universal asynchronous receiver and transmitter serial communications interface (UART/SCI) hardware that is shared by most micro-controllers, for a more flexible, lower-cost silicon implementation.
The wires of a communication bus or similar line are often long and present a substantial distributed reactive load to the transmitter to which they are connected and especially their capacitive loads may be individually variable. The distributed impedance gives the wave fronts of a nominally rectangular switched pulse a finite slew rate. It is accordingly important for the receiver to respond at an accurately repeatable signal level in order to ensure accurate timing of the receiver response. This is important for a CAN bus and other systems but the self-synchronisation feature of a LIN system makes it especially important for the response level of a LIN receiver to be precise.
It is also important for the standby current of the nodes of the system to be very low, especially where such systems are powered by a battery or accumulator. Accordingly, the nodes of the system have standby modes of operation, in which current consumption is reduced but it is also desirable for the wake-up time, that is to say the time taken to pass the node from the standby to the operational mode to be short. In addition, cost considerations are important and make it desirable for components of the nodes to use as small an area of silicon as possible; it follows that it is preferable to avoid including extra receiver components to detect the signal front and wake up main receiver elements in the node.
U.S. Pat. No. 6,281,714 discloses a differential receiver circuit for computer and other information processing systems but does not disclose a receiver for a communication bus system enabling standby current of a node of the system to be reduced.